The development of radioimmunoassay (RIA) for prolactin (PRL) and sensitive high-resolution computed tomography (HRCT) has resulted in documentation of large numbers of microprolactinomas (microPRL). To try to define the rate and prevalence of tumor growth, the authors performed serial HRCT scans in 38 untreated patients, an average of 31.7 months apart. In no patient, including two patients with exuberant rises in PRL, was there any evidence of subtle tumor growth. The authors conclude that the majority of patients with microPRL have a benign course, with tumor growth occurring, if at all, over a period of years.